Knobs for machining at new heights

More than most industries, aerospace component manufacturers machine a lot of exotic alloys and challenging metals like titanium. A byproduct of using these materials is that more pressure is put on machines, spindles and retention knobs, causing cutting tools to be changed more often as they become dull or break. Tool replacement, not to mention loss of production time due to frequent changeovers, can escalate costs as well.

Founded in 2001, Flying S Inc. in Palestine, Illinois, machines parts for the aerospace industry and has continued expanding with more palletized five-axis machining centers to handle the high volume of complex, tight-tolerance parts made of titanium, stainless steel and other difficult-to-machine materials. Notably, as the five-axis parts that the company made for unpiloted military and civilian aircraft, along with spacecraft, became more complicated, it was clear that greater machine capabilities would be needed to achieve reliable, unattended operation.

“Many of our spaceflight fittings require tight-tolerance features,” said Manufacturing Engineer Peter Bowman. “Our lack of reliable positional accuracy and repeatability due to thermal growth, machine positioning or other variables really took a toll on our efforts to consistently make a good first part and subsequent parts.”

With new five-axis machining centers in place, including models from Matsuura Machinery USA Inc. and Brother Industries Ltd., significantly higher torque and drawbar pressure were being exerted on standard retention knobs, causing issues with the interface between the toolholder and spindle. Standard retention knobs kept coming loose as dimpling and grooving wear patterns repeatedly appeared on the face of knobs, requiring regular replacement.

At a trade show, Bowman witnessed an expanding taper demonstration by Fairport Harbor, Ohio-based JM Performance Products Inc., which featured its patented High Torque retention knobs. They were engineered to eliminate the toolholder expansion responsible for costly, ongoing CNC milling and boring issues. JMPP’s retention knobs feature a relief beneath the precision pilot that forces the threads of the knob deeper into the threaded bore of the toolholder, where a thicker cross section of material resists expansion and deformation.

With the expansion into multiple-​axis and unattended operations in full force, Flying S set out to standardize its entire line of CNC machines with JMPP’s retention knobs to overcome catastrophic failure because of metal fatigue and other high-load, force-related issues associated with difficult-to-machine materials and long-reach tools.

Today’s CNC mills, such as Flying S’ multiple-axis machining centers, are designed to operate with high drawbar pressure, a large depth of cut, fast tool changes and high feed rates, which place more demand than ever on retention knobs in V-flange holders. Considering that spindle speeds in excess of 20,000 rpm often are required, a unique set of production machining challenges ensues. For instance, vibration and chatter can occur at these high rates of speed, leading to poor surface finishes that need secondary bench operations or result in serious damage to expensive workpieces.

Any one element could impede the effectiveness of the overall process as the tool, toolholder, spindle, column, ways, table, fixturing and workpiece are all part of the interconnected “machining circle” that necessitates stability to achieve optimal results. According to JMPP, retention knobs are the critical connection between a machine tool and a toolholder, and they are the only thing holding a steep-taper toolholder in the machine spindle.

However, the company reports that many machine shops frequently overlook retention knobs, with a greater focus placed on having high-quality machine tools, toolholders and carbide cutters. Then, ironically, these shops break the loop by using standard retention knobs. Some shops go through thousands of tool changes, which means that knobs are subjected to the high pull forces from the spindle’s drawbar. Excessive torque tends to swell the metal around the retention knob, distorting the toolholder and creating an imperfect fit between it and the spindle taper. Additionally, toolholder expansion caused by standard retention knob installation can reduce the toolholder-to-spindle contact by 70% or more.

Nonetheless, an expensive retention knob does not guarantee better performance. JMPP reports that it redesigned the standard retention knob into the High Torque retention knob, which uses the lowest available threads in the toolholder. The pilot portion beneath the knob flange above the threads stabilizes the knob in the toolholder. The High Torque design includes a precision-ground pilot that ensures exact alignment of the knob in the holder during installation.

The company also manufactures all 30- and 40-taper retention knobs, including High Torque knobs, from AISI 9310H material, which offers 40% more tensile strength than the traditional 8620H material.

Deep-pocket drilling of materials, such as titanium, for example, requires more coolant application, which acts like a high-pressure garden hose. This through-spindle coolant, or TSC, runs through a hole in the knob, which allows the coolant to go through the tool to the tip of the drill, helping to achieve heavy cuts at high feed rates while imparting fine surface finishes. It also enables better chip evacuation by forcing chips out of the hole, improving process reliability.

JMPP’s longer length and shorter threads versus standard knobs mean less torque value and more leverage on the holder for high-pressure coolant use with less fretting on the holder or vibration. The coolant holes also have been sized to increase the cross-sectional strength of the knob.

“We use JMPP’s knobs as standard for both TSC and non-TSC applications across the board because there is no need to swap them out when using TSC,” Bowman said. “It’s a safety measure for us. We would rather standardize so that all retention knobs are TSC-capable, thus reducing the chance of trying to run high-pressure coolant on a non-TSC knob and flooding our spindles. What we found is that the knobs handle the force on the holder exceptionally well with no leaks at 1,000 psi (68.9 bar) and no signs of excessive wear.”

With the increased number of multiple-axis machining centers, Flying S has implemented a standardization plan to exclusively use JMPP’s retention knobs. To date, the company has roughly 2,000-plus retention knobs in operation, including small-spindle, high-production five-axis machines — typically with longer tool requirements and robotically loaded — so the retention knobs work extremely hard. Most of Flying S’ tooling is CAT 40, so it is midsized.

“We’ve had no issues as far as I’m concerned and will only use JMPP knobs going forward,” Bowman said.

Machining complex aerospace parts is common, with more companies investing in multiple-axis machining centers that can match the demanding production challenges. By using more than 2,000 High Torque retention knobs, every tool on Flying S’ machines helps reduce machine downtime, extend tool life and increase profit margins.

“We were finding ourselves replacing the older $11 knobs on a regular basis,” Bowman said. “However, I have never replaced a single JMPP knob. Our standardization investment is great, and I think it will last. We had no idea how much our ‘economy’ retention knobs were costing us.”